sync: multi-branch scanning, discovery strategies, UTF-8 fix, curation improvements

- Add multi-branch scanning to discover skills on non-default branches
- Add popular repos and commits search discovery strategies
- Fix UTF-8 encoding sanitization in skill parser
- Add repo_created_at for threshold-based duplicate tie-breaking
- Add indexer tests and vitest config

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/schema.ts

@@ -102,6 +102,7 @@ export const skills = pgTable(
     // Timestamps
     createdAt: timestamp('created_at').defaultNow().notNull(),
     updatedAt: timestamp('updated_at').defaultNow().notNull(),
+    repoCreatedAt: timestamp('repo_created_at'),
     indexedAt: timestamp('indexed_at'),
     lastDownloadedAt: timestamp('last_downloaded_at'),
   },